Current beneficiation practices for phosphate ores commonly utilize flotation as the principal step for separating phosphates from gangue minerals.
Most natural deposits of phosphate rock, such as those found in Florida, contain an objectionably high percentage of silica and silicates. Typical Florida ore as mined will contain approximately one-third phosphate mineral, one-third silica or siliceous gangue, and one-third clay. In the processing of these ores, they are first subjected to washing and screening operations in the "washer plant" to remove the clay constituents as slime and to recover coarse, pebble concentrate. Deslimed undersize (essentially -14 mesh to +150 mesh) from the screening operation is further separated at about 28 to 35 mesh. The coarse fraction is conditioned with tall oils and fuel oils and treated on concentrating tables, spiral concentrators, or spray belts. The fine mineral fraction is subjected to the Crago or "double-float" process which utilizes two stages of froth flotation. In the first stage, the flotation feed is conditioned in the anionic circuit with either caustic soda or NH.sub.3, as well as fuel oil, tall oil mixtures of fatty acids, or oleic acid. The conditioned feed is then subjected to froth flotation where phosphates are floated and the underflow is discarded to waste. The product obtained from this flotation operation normally still contains so much silica that further treatment is necessary.
Accordingly, this intermediate product is de-oiled by scrubbing with sulfuric acid followed by desliming. The de-oiled, deslimed product is then subjected to a second stage of froth flotation in a cationic circuit with amines, where the silica is floated and discarded to waste. The underflow of the second stage of flotation is the final phosphate product.
However, some phosphate ores of potential commercial value contain surface-activated silica. This phenomenon results in unacceptable amounts of quartz silica sand being floated along with phosphate minerals in the fatty acid flotation circuit, which subsequently results in a heavy loss of phosphate minerals in the amine flotation circuit of the conventional "double float" process, supra.
The problem of surface-activated silica may occur naturally in phosphate ores, which are subjected to the geological or geochemical alternation process with underground water, such as leaching of source rock and adsorption or precipitation of metal ions, hydroxides, sulfate, or other chemical species on the mineral surface. The problem may also result from the contamination of surface water or processing water which may contain such cationic species as Ca, Mg, Al, or Fe, before or during the beneficiation process. These cations or cationic derivative species may activate silica surfaces so as to be responsive to fatty acid collectors, which subsequently results in an excessive amount being floated along with phosphate in the fatty acid flotation step in the two-stage process.
Phosphate ores which are beneficiated by the method of the present invention may be classified to desired size range by methods known in the art. A particle size smaller than about 28 mesh is preferably used for the flotation process. With larger particle size, an appreciable quantity of the gangue minerals may remain locked with the apatite. Furthermore, the larger particles are sometimes difficult to float. Very small particles, e.g., smaller than about 400 mesh, are removed by a desliming process. Although the slime may contain phosphate values, their relatively large consumption of reagents makes their beneficiation economically unattractive in relation to recoverable values of P.sub.2 O.sub.5.